The present embodiments relate to a method for the wireless transfer of energy to a local coil system for a magnetic resonance system.
Imaging with a magnetic resonance tomography system is carried out essentially in three steps. Initially, a strong, stable homogeneous magnetic field, and thus a stable orientation of the nuclear spin of the protons, is generated in a region under examination (e.g., a region of the body of a patient). This stable orientation is then altered by feeding in high frequency electromagnetic energy (e.g., magnetic resonance excitation signals or MR excitation signals). This may be achieved with a whole-body antenna arranged fixed in the magnetic resonance tomograph (e.g., a “birdcage” antenna) that surrounds the examination space, in which the patient is situated during the examination. The energetic stimulation is ended, and the magnetic resonance signals (e.g., magnetic resonance detection signals or MR detection signals) arising in the examination region on relaxation of the nuclear spin are detected with the aid of suitable detection antennae (e.g., detection coils) in order to be able to draw conclusions about the material or tissue in the examination region. On excitation and/or acquisition of the MR detection signals, rapidly switched magnetic field gradient pulses are transmitted with a gradient coil system for position encoding. A magnetic resonance tomography system therefore includes a plurality of cooperating components, each of which uses of modern and complex technologies.
In a magnetic resonance examination, “local coils” may be used to receive the MR detection signals, so as to achieve the best possible signal-to-noise ratio (SNR). The local coils include detection antenna assemblies that have at least one detection antenna element (e.g., a plurality of detection antenna elements) in the form of, for example, conductor loops that are mounted closely on, under or at the patient. The local coils may be arranged in a “local coil mat” that is laid over or under the body of the patient. In many examinations, a plurality of local coils of this type is arranged at the patient to cover whole regions of the body of the patient. The MR detection signals received may be pre-amplified in the local coil and are conducted out of the central region of the magnetic resonance system via cables and fed to a screened receiver of an MR signal processing device. In the device, the received data are then further processed.
The structure and function of magnetic resonance systems is known and is described, for example, in Imaging Systems for Medical Diagnostics, edited by Arnulf Oppelt, Publicis Corporate Publishing, ISBN 3-89578-226-2, and further detailed discussion is unnecessary.
Due to the MR excitation signals for stimulation of nuclear spin, the patient is subjected to a physiological high frequency exposure. During imaging, it is to be provided that the patient is not harmed by such physiological high frequency exposure. In the development and establishment of magnetic resonance tomography systems, therefore, limit values that define the maximum physiological high frequency irradiation of a human body or the high frequency exposure of the human body are standardized to provide patient safety. A typical limit value for this is the maximum admissible specific absorption rate (SAR) value For example, it is required for the “whole body SAR” that the power absorbed by the patient averaged over a time window of 6 minutes is to not exceed a value of 4 W/kg with physiological monitoring. In magnetic resonance systems, measuring devices, with which the high frequency power may be measured, are provided. For this purpose, directional couplers may be used in the feed lines from a high frequency amplifier to the antenna system in order to transmit the MR excitation signals.
The signals from the local coils are transmitted from the local coils by cables to an evaluating device of the magnetic resonance system. The cables are undesirable because the cables cannot easily be fed from the patient support to the evaluating device, since the patient support is moved with the patient, and the local coil mat and consequently the cables hang loosely. The cables are therefore often found to be a nuisance.
In order that no cables are connected to the local coil mat, the received MR detection signals are transmitted to the MR signal processing device and power supply to the local coils are provided wirelessly. In order to transfer energy to the local coil mat, for example, inductive methods may be used. The inductive transfer of energy to the local coil produces additional physiological high frequency exposure of the human body. Consequently, the physiological high frequency exposure due to the actual imaging is kept low if energy is also transferred wirelessly to the local coil system, in order that the maximum permissible physiological high frequency exposure of the patient is not exceeded by the imaging and the wireless energy supply.